The present invention is directed to an apparatus for determining the location of an object along one or more axes and in particular to an apparatus for accomplishing such location by optical means.
In today's world the increasing prevalence of computers and devices using computer-like apparatus has given rise to a recognition of a need for simplifying the human operator-to-computer interface to facilitate the input of data to the computing apparatus. Numerous devices and apparatus have been produced for such interface enhancement such as keyboards, joystick controls, and various types of touch screen inputs. The various types of touch screen inputs include screen overlays superimposed upon cathode ray tube displays of computing devices, which apparatus can be capacitive, resistive, ultra-sonic or consist of a conductive grid. There are various disadvantages to such touch screens: the overlay in front of the cathode ray tube degrades the contrast of the display as it appears to an operator and may also degrade the resolution of the display on the cathode ray tube, such overlays attract and trap dirt to further contribute to degradation of contrast and resolution, such overlays are often made of materials which are easily scratched or otherwise optically degraded with time or use, and with such overlays non-glare cathode ray tube finishes are greatly reduced in effectiveness. Moreover, such overlays touch screens generally increase in complexity and expense of manufacture with increases of resolution in their detection capabilities.
Optical touch screen input apparatus overcome the aforementioned shortcomings of touch screen overlays by creating a light curtain in front of a cathode ray tube or other display, penetration of which curtain is detectable by the apparatus and interpreted to fix the location of the penetration in the requisite number of dimensions for the particular application. Examples of types of optical touch screen input devices include the system disclosed in U.S. Pat. No. 4,267,443 (Carroll) and the device disclosed in U.S. Patent Application Ser. No. 183,357, filed Sept. 2, 1980 (Barlow). The Carroll system employs arrays of light emitting diodes and photo-detectors along opposite sides, switching of the light emitting diodes in a sequential manner and sensing the presence of light sequentially in the opposing photo-detectors. With appropriately situated arrays of light emitting diodes and corresponding opposing photo-detectors it is possible by such a device to determine the location of an object within a two dimensional location field. The Carroll apparatus, however, is disadvantageous because the large numbers of light emitting diodes and photo-detectors required by that apparatus render it expensive to construct. Further, the large number of discreet elements renders the device more prone to material breakdown than a device with a lesser number of components. Still further, resolution of the Carroll apparatus is limited by the number and size of the light emitting diodes and photo-detectors, and any increase in resolution of detection by such a device is necessarily accompanied by a commensurate increase in the number of components, thereby increasing the cost of construction and the probability of breakdown.
The Barlow device employs a single continuous light source in a corner of a touch field and a photo-detector scanning the touch field, which photo-detector is located in the opposite corner from the light source. Along all four sides of the touch field there are stepped mirrors arranged so that light from the light source is reflected across the touch field in two perpendicular arrays and subsequently directed toward the rotating detector. Thus the light from each beam arrives at the detector from a slightly different angle and, because the detector is rotatively scanning the touch field, the pattern of light of the detector is interpretable to determine the location of an object within the touch field in two dimensions. Such a device as is disclosed by Barlow overcomes some of the disadvantages of Carroll in that it is potentially lower in cost of production and maintenance, however, a high degree of precision is required in building a workable Barlow system since the mirrors must be precisely aligned for proper operation to occur. Such a requirement for a high degree of precision in alignment necessarily adds to cost of production and sensitivity to physical shock, both of which are disadvantageous in a commercial environment.